Dinuclear Zinc(II) Complexes of Tetraiminodiphenol Macrocycles and Their Interactions with Carboxylate Anions and Amino Acids. Photoluminescence, Equilibria, and Structure

The reaction equilibria [H4L]2+ + Zn(OAc)2 ⇌ [Zn(H2L)]2+ + 2HOAc (K1) and [Zn(H2L)]2+ + Zn(OAc)2 ⇌ [Zn2L]2+ + 2HOAc (K2), involving zinc acetate and the perchlorate salts of the tetraiminodiphenol macrocycles [H4L1-3](ClO4)2, the lateral (CH2)n chains of which vary between n = 2 and n = 4, have been studied by spectrophotometric and spectrofluorimetric titrations in acetonitrile. The photoluminescence behavior of the complexes [Zn2L1](ClO4)2, [Zn2L2(H2O)2](ClO4)2, [Zn2L2(μ-O2CR)](ClO4) (R = CH3, C6H5, p-CH3C6H4, p-OCH3C6H4, p-ClC6H4, p-NO2C6H4), and [Zn2L3(μ-OAc)](ClO4) have been investigated. The X-ray crystal structures of the complexes [Zn2L2(H2O)2](ClO4)2, [Zn2L3(μ-OAc)](ClO4), and [Zn2L2(μ-OBz)(OBz)(H3O)](ClO4) have been determined. The complex [Zn2L2(μ-OBz)(OBz)(H3O)](ClO4) in which the coordinated water molecule is present as the hydronium ion (H3O+) on deprotonation gives rise to the neutral dibenzoate-bridged compound [Zn2L2(μ-OBz)2]·H2O. The equilibrium constants (K) for the reaction [Zn2L2(H2O)2]2+ + A- ⇌ [Zn2L2A]+ + 2H2O (K), where A- = acetate, benzoate, or the carboxylate moiety of the amino acids glycine, l-alanine, l-histidine, l-valine, and l-proline, have been determined spectrofluorimetrically in aqueous solution (pH 6−7) at room temperature. The binding constants (K) evaluated for these systems vary in the range (1−8) × 105.